Ceramic cutting insert of polycrystalline tungsten carbide

ABSTRACT

A polycrystalline tungsten carbide ceramnic cutting insert with chip control is disclosed for high speed machining.

FIELD OF THE INVENTION

[0001] This invention relates to the field of ceramics and particularlyto dense polycrystalline tungsten carbide inserts with chip control.

BACKGROUND OF THE INVENTION

[0002] In the machining process, it is important for the cutting tool towork effectively at high speeds and to have a long tool life. In orderfor the cutting tool to be effective, it must be made of a materialwhich results in the tool having a high heat hardness and a hightransverse rupture strength and fracture toughness, and it must alsohave a design sufficient to control the flow of chips which are formedin the machining process and to reduce the cutting forces.

[0003] Chip control is an important element of the machining process, inorder to break up the length of undesirably long chips which may beformed in the machining process. In high speed machining, if the striptaken off from the workpiece by the cutting insert is not broken up, thestrip can interfere with the machining process in a variety of ways. Forexample, an undesirably long chip can be re-cut and welded onto aportion of the workpiece, thereby causing poor surface conditions on theworkpiece. An undesirably long chip, if not broken under chip control,can also cause breakage of the machining tool itself. Additionally,undesirably long chips can feed into the tool holder or other portionsof the machine and cause difficulties, e.g., damaging parts of the toolholder or obstructing visibility of the working area. Further, longribbons are difficult to handle and can represent a safety hazard to themachine operator. Accordingly, there is a need in the high speedmachining process to provide chip control. One method for controllingchip production is to incorporate an insert into the cutting tool, withthe insert providing the means for chip control. Many different types ofceramic cutting tools with chip control inserts have been described,including those in U.S. Pat. Nos. 5,628,590; 5,141,367; and 5,330,296,the contents of which are hereby incorporated in their entirety.

[0004] In addition to chip control, another important aspect of cuttingtools are the materials of which they are made. Cutting tools have beenmade with ceramics and ceramic-metal composites (“cermets”), includingtungsten carbide (“WC”). Early work with WC focused upon densifying WCby heating to a temperature of, for example, 2,000 C°. The densifiedmaterial was judged unsuitable for use in applications requiringtoughness, such as in cutting tools. The unsuitability stemmed largelyfrom the densified material's excessively brittle character.

[0005] Efforts to overcome or offset some of the brittleness led toincorporation of an amount of a metal by admixing powdered metal and WCpowder to form a composite and densifying the composite at a temperatureabove that at which the metal melts. The metal, most frequently an irongroup metal (iron, cobalt or nickel), was added to impart some of itsductility to the composite. The densified composites, also known ascemented carbides, cermets and hard metals, have been used extensivelyfor several decades in machining tools. In order to increase the cuttingspeed and cutting efficiency, a variety of additions have been made tothe composition of ceramic cutting tools.

[0006] In general, hardness of the cermets,(i.e., wear resistance andstrength and toughness, i.e., fracture resistance of a hard alloy) canbe changed by tungsten carbide particle size, cobalt content andadditional amounts of other carbides. The resulting hardened alloy hasbeen widely used for various purposes. However, in formulating thesematerials, there is a tendency that if wear resistance is heightened,fracture resistance is lowered, and conversely, if fracture resistanceis heightened, wear resistance is lowered. Therefore, in the design ofcermet cutting tools, there has been encountered the problem ofimproving one material property at the expense of another materialproperty by adding cobalt or another iron group that will plasticallydeform in the heat of high speed machining.

[0007] There have been many attempts to solve this problem, includingremoving the machining equipment from use and reprofiling the cementedcarbide cutting tool in order to reestablish its desired properties andscrapping the used cemented carbide portion and inserting a new cementedcarbide portion with the desired properties. There currently exists aneed for machining tools with chip control which can maintain thedesired machining properties of wear resistance and breakage resistanceduring high heat high speed machining.

[0008] Although cermets and WC have been used extensively in the designof cutting tools, there still has not been a satisfactory resolution tothe problem of tailoring the composition of the cermet or WC in order tomaximize efficiency of the cutting tool. The present invention solvesthis problem by incorporating into a machining tool a chip controlinsert made of WC. Such inserts have not been previously used, and suchinserts maximize the efficiency of machining tools.

[0009] SUMMARY OF THE INVENTION

[0010] Cutting tool inserts with chip control composed of essentiallydense, fine grained polycrystalline, tungsten carbide (WC) are includedin the present invention.

[0011] According to the present invention, the ceramic cutting insertfor high speed machining includes a cutting edge, a rake face with achip control groove surface, a flank face and the cutting edge that isformed at the juncture of the flank face and the rake face.

[0012] These components are made by ceramic processing techniques andresult in a ceramic body which has a density of greater than 95% of itstheoretical density with substantially all grains having an average sizeof 0.001 to 20 micrometers.

[0013] In other aspects the insert is comprised of polycrystallinetungsten carbide of at least 98.5% by volume tungsten carbide. Inanother aspect an iron group, e.g. cobalt is present in the ceramic bodyfrom 0.01% to 1.5% by volume.

[0014] A further aspect of the present invention is to control the grainsize of the tungsten carbide by adding an inert second phase such asrefractory oxides, carbides, nitrides or borides.

[0015] The ceramic articles of this invention are particularly useful aswear parts, especially as cutting tools for a wide variety of materials,including the machining titanium metals and alloys of titanium whichhave a very high content of titanium, cast iron, aluminum, high nickelalloys, stainless steels, wood machining-cutting, and high speedmachining of steels.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1. is an illustration of types of chips that can be formedduring machining. Panel A illustrates the chips formed using a cuttingtool with no chip control. Panel B illustrates the chips formed using acutting tool with chip control. Panel C is an illustration of the typesof chips formed when high speed machining.

DETAILED DESCRIPTION OF THE INVENTION

[0017] It is the primary object of the invention to make a moldedpolycrystalline tungsten carbide ceramic cutting insert having a chipcontrol structure. In drilling operations, chip control is important sothat drilling efficiency and tool insert damage does not result. (SeeFIG. 1) Incorporating chip breaker grooves or lands on the cuttinginsert blank allows the strip taken off of the workpiece to be broken upinto short pieces. These small chips will readily fall away from themachining region into a receiving space or containers, so that the chipsare contained and can be removed from the machine tool.

[0018] Previously, polycrystalline tungsten carbide has not beenutilized in the preparation of cutting tool inserts with chip controlbecause tungsten carbide was not thought to have the toughness, i.e.fracture resistance, and hardness, i.e., wear resistance, required forinserts with chip control. Toughness and hardness can be altered by theaddition of an iron group such as cobalt. However, cobalt melts at thehigh heat of high speed machining, making these formulations unsuitablefor the manufacturing inserts with chip controls. Surprisingly, byvarying polycrystalline tungsten carbide powder size, percentage ofcobalt, temperature and pressure, the present invention provides apolycrystalline tungsten carbide formulation capable of being molded(i.e., in punches and dies) into cutting tools with chip control thathave increased toughness and hardness suitable for high speed machining.The present invention may be utilized with a variety of insert chipcontrol designs, such as those shown in U.S. Pat. Nos. 5,141,367;4,318,645; 4,340,324; 4,247,232; 4,087,193; 4,056,871 and 3,383,748.

[0019] U.S. Pat. Nos. 5,563,107 and 4,828,584 include various examplesof tungsten carbide ceramic materials which have been utilized in thepreparation of cutting tools and are incorporated herein by reference.Until the present invention, however, such materials were not used inthe production of inserts with chip control.

[0020] Tungsten carbide (WC) ceramics of the present invention can betailored for use in particular applications by an appropriate choice ofstarting WC powder size and by controlling densification conditionscontrol grain growth.

[0021] Desirable starting powder sizes fall within a range of fromgreater than 0.001 μm up to 20 μm. The range, depending on application,is preferably from about 0.001 μm to about 10 μm, more preferably fromabout 0.001 to about 4 μm. In one embodiment, the tungsten carbidepowder size is about 1.0 μm. Starting powder sizes of less than 20 μmshould provide densified bodies having excellent properties.

[0022] Tungsten carbide powders having an average particle size of lessthan or equal to 10 μm are commercially available. One such powder,Teledyne type IV, has a nominal average particle size of 8 μm andincludes a small amount of vanadium carbide as a grain growth inhibitor.Attriting such a powder simultaneously reduces the average particlesize, reduces grain size distribution, and more uniforrnly disperses thegrain growth inhibitor. Even in the absence of a grain growth inhibitor,attrition provides the benefits of smaller average particle size and anarrower particle size distribution. As an alternative, the WC powdermay have these characteristics as synthesized. As a further alternative,powders with even larger average particle sizes may be used providedthey are milled or attrited under conditions sufficient to reduce theaverage particle size to less than or equal to 0.2 μm. These powdersnecessarily require longer size reduction procedures and may, as aconsequence, pick up additional quantities of impurities from media usedto promote size reduction.

[0023] WC powders used in the present invention need not be 100% pure.They may contain very small amounts, e.g., less than 1.5 wt % by volume,of other materials so long as the other materials do not interfere withdensification of the powder or adversely affect physical properties ofresultant densified bodies. Examples of “other materials” includecobalt, iron, nickel, carbon and silicon. The other materials may, forexample, be present as a result of powder synthesis procedures or asresidue from milling operations. In some embodiments, cobalt is presentfrom about 0.01% to 1.5% by volume. Preferably cobalt is present atabout 0.25%. In addition to the other materials, the WC powders have anoxygen content that varies inversely with particle size. Thus, asparticle size decreases, oxygen contents tend to increase. However, theoxygen content should be maintained at a level that does not interferewith densification of the powder or adversely affect physical propertiesof resultant densified bodies. In some embodiments a binder, e.g., waxis added to the powder to facilitate molding into the die. Preferably,the binder is less than about 5% by volume. More preferably the binderis about 2.25% by volume. Grain size can be controlled by carefulcontrol of densification procedures even if the WC powder does notinclude a grain growth inhibitor. Any conventional densificationtechnique may be used provided it yields the densified ceramic body ofthe invention. Conventional techniques include pressureless or lowpressure sintering, hot pressing, hot isostatic pressing and rapidomnidirectional compaction. Densification is preferably accomplished byhot isostatic pressing.

[0024] Hot pressing of essentially pure WC powders at temperatures lessthan or equal to 1,700° C. and pressures of 35 MPa has resulted inpolycrystalline tungsten carbide bodies which are greater than 98.5% oftheoretical density. Also, significant densification, (linear shrinkageof 9%) has been observed for essentially pure but agglomerated WCpowders sintered without external pressures at 1,600° C. for 30 minutesin Ar. Such significant solid state sintering occurs in the absence ofadditives, although small percentages of an iron group element, e.g.cobalt, resulted in the attainment of closed porosity by pressurelesssintering and near theoretical density (greater than 98% theoreticaldensity). When such powders contain very small amounts of such additivesare hot pressed at temperatures between about 1,400° C. and 2,000° C.,the result in a fine-grained microstructure with isolated pores.Preferably, the WC powders are pressed at about 1900° C. and about50,000 psi.

[0025] Additionally, grain size can be controlled by very carefulcontrol of the processing conditions, especially sintering conditions,and by adding a small amount of a second phase which inhibits graingrowth. Suitable grain growth inhibitors must be compatible with WC,eg., oxides, carbides nitrides or borides. The free energy of oxidesused as grain growth inhibitors must be lower than WO₂ and WO₃ (i.e.AL₂O₃, ZrO₂, TiO₂, NbO, NbO₂, Nb₂O₅, Cr₂O₃, MgO, SiO₂, Ta₂O₃, MnO, ZnO,ThO₂, BeO etc.) Suitable carbides for controlling grain size must havefree energies of formation less than WC (i.e. VC, ThC₂, Cr₂₃, C₆, ZrC,TiC, SiC, Cr₃C₂, etc.). Preferably, VC is added to the WC powder.Nitrides used as grain growth inhibitors must have free energies offormation less than WN₂, and WN (i.e., ZrN, TiN, Th₃N₄, AlN, BN, NbN,VN, Si₃N₄, Cr₂N, etc.). Similarly, borides must have energies offormation less than WB₂, WB, and W₂B, (i.e. ZrB₂, TiB₂ etc). Such graingrowth inhibitors are present as less than about 1.5% by volume of theceramic body with quantities less than about 1% volume percent beingpreferred, and about 0.35% by volume being especially preferred.

[0026] In all the above illustrations, it is necessary to realize thatother interactions may also occur. For example, the addition of TiC,TIN, or TiG promotes the formation of cubic WC in solid solution withTiC, TIN or TiO and therefore changes the microstructure

EXAMPLE 1

[0027] Tungsten carbide powder (particle size of 1 micron), 0.35% VC and2.25% wax was spray dried into a pressable powder. The powder waspressed in a cavity with punches and dies to have to form of the insertshape and chip breaker put into the ceramic body at the same time. Theceramic cutting insert was heated to 400° C. under argon to remove thewax binder. After all traces of wax binder was gone the insert washeated to 1900° C. until parts became dense. While maintainingtemperature pressure (50,000 psi) was applied to remove porosity.

[0028] Other Emdodiments

[0029] It is to be understood that while the invention has beendescribed in conjunction with the detailed description thereof, theforegoing description is intended to illustrate and not limit the scopeof the invention, which is defined by the scope of the appended claims.Other aspects, advantages, and modifications are within the scope of thefollowing claims.

What is claimed is:
 1. A ceramic cutting insert for high speed machining consisting essentially of polycrystalline tungsten carhbide comprising: (a) a cutting edge; (b) a rake face with a chip control groove surface; (c) a flank face; wherein the cutting edge is formed at the juncture of the flank face and the rake face.
 2. The ceramic cutting insert of claim 1, wherein the flank face is in an as molded or ground condition.
 3. The ceramic cutting insert of claim 1, wherein the chip control groove surface is in an as molded or ground condition.
 4. The ceramic cutting insert of claim 2, wherein the chip control groove surface and the flank face are each in an as molded condition.
 5. The ceramic cutting insert of claim 1, wherein the polycrystalline tungsten carbide: (a) has a density of at least 95% of its theoretical density; and (b) an average grain size of 0.001 to 10 micrometers.
 6. The ceramic cutting insert of claim 5, wherein the polycrystalline tungsten carbide has an average grain size of 0.2 to 4 micrometers.
 7. The ceramic cutting insert of claim 5, wherein the polycrystalline tungsten carbide comprises at least 98.5% by volume tungsten carbide.
 8. The ceramic cutting insert of claim 5, wherein the polycrystalline tungsten carbide comprises 99% by volume tungsten carbide.
 9. The ceramic cutting insert of claim 5, wherein the polycrystalline tungsten carbide has the density of at least 98% of its theoretical density.
 10. The ceramic cutting insert of claim 5, wherein an iron group element is present from about 0.01% to about 1% by volume.
 11. The ceramic cutting insert of claim 10, wherein the iron group element is cobalt.
 12. The ceramic cutting insert of claim 11, wherein cobalt is present from about 0.01% to about 1.0% by volume.
 13. The ceramic cutting insert of claim 11, wherein the cobalt is present at about 0.25% by volume.
 14. The ceramic cutting insert of claim 5, wherein a carbide, nitride, oxide and/or boride have a free energy of formation less than that of the respective carbide, nitride, oxide and/or boride of tungsten is present in the amount from about 0.1% to about 1% by volume. 